1. Field of the Invention
The present invention relates to an air conditioning apparatus and a method of controlling thereof. In particular, the invention relates to an air conditioning apparatus in which refrigerating machine oil can be returned sufficiently, and to a method of controlling thereof.
2. Description of the Background Art
A conventional air conditioning apparatus is constituted as shown in FIG. 1. Specifically, the air conditioning apparatus is composed of a compressor 21, a four-way valve 22, an interior side heat exchanger 23, an expansion device 24, an exterior side heat exchanger 25, a refrigerant pipe 26, an interior side blower 27 and an exterior side blower 28.
In this configuration, refrigerant flows as shown by the arrow of the solid line in the case of cooling.
Specifically, the refrigerant discharged from compressor 21 passes through four-way valve 22 and is condensed by exterior side heat exchanger 25. After the heat is discharged into the exterior air, the temperature and pressure of the refrigerant are reduced by expansion device 24. The refrigerant flows into interior side heat exchanger 23, cools the interior air, and evaporates. The evaporated and vaporized refrigerant is passed through four-way valve 22, and sucked into compressor 21.
In the case of heating, the refrigerant flows as shown by the arrow of the dotted line. Specifically, the refrigerant discharged from compressor 21 is passed through four-way valve 22, and condensed by interior side heat exchanger 23. The refrigerant heats the interior air, the temperature and pressure of the refrigerant are reduced by expansion device 24, and the refrigerant flows into exterior side heat exchanger 25. The refrigerant receives heat from the exterior air and evaporates. The evaporated and vaporized refrigerant is passed through four-way valve 22, and sucked into compressor 21.
The refrigerating machine oil is enclosed in compressor 21 for lubrication of the mechanical section. The apparatus has a mechanism by which the refrigerating machine oil is difficult to let flow from compressor 21. However, currently a small amount of refrigerating machine oil is emitted with the refrigerant from compressor 21. Since refrigerant flon (chlorofluoro carbon) 22 currently used by the air conditioning apparatus has compatibility with the refrigerating machine oil, the refrigerating machine oil is melted into the refrigerant, passed through interior side heat exchanger 23 and exterior side heat exchanger 25, and returned to compressor 21. Accordingly, the refrigerating cycle proceeds without trouble.
Due to the problem of destruction of ozonosphere, flon 22 which is HCFC type flon is to be successively reduced during the period from 2004 to 2020. Therefore, the flon should be replaced with HFC type flon or the like of which modulus of destruction of ozonosphere is zero. Three-component type refrigerant R407C (R32/R125/R134a:23/25/52 wt %), two-component type refrigerant R410A (R32/R125:50/50 wt %) are the HFC type flon that are leading substitutes as the refrigerant. However, the compatibility of the substitute refrigerant with mineral oil used as the conventional refrigerating machine oil is poor, and the rate of the refrigerating machine oil emitted from compressor 21, mixed with the refrigerant and returned to the compressor becomes extremely low. As a result, the amount of the refrigerating machine oil in the compressor could be insufficient to cause inadequate lubrication.
As a refrigerating machine oil which has the compatability with the substitute refrigerant described above, refrigerating machine oil of polyester type or polyether type is developed. Although the polyester type refrigerating machine oil has hygroscopicity and is easy to be hydrolyzed, the sludge is often found in case of contamination of any impurities such as the machine oil. Further, although the polyether type refrigerating machine oil has the hygroscopicity, there are unknown factors of the cost and the possibility of supply. Consequently, the refrigerating machine oil is selected out of the conventional mineral oil, the polyester type refrigerating machine oil, the polyether type refrigerating machine oil, and the like. Whichever oil is selected, the compatability with the refrigerant would be poor, and any measures to return the refrigerating machine oil are necessary.
If returning of the refrigerating machine oil is inadequate in a large-sized air conditioning apparatus or the like, the configuration as shown in FIG. 2 is conventionally used. Specifically, a by-pass formed of an oil separator 29 and a restrictor 30 is provided between a delivery outlet and a suction inlet of compressor 21, the refrigerating machine oil contained in the refrigerant discharged from compressor 21 is separated by oil separator 29, the separated refrigerating machine oil is passed through restrictor 30 and sucked into compressor 21. The capillary tube or the expansion valve is used as restrictor 30. If the resistance of flow passage is too high, the refrigerating machine oil does not flow sufficiently. If the resistance of flow passage is too low, a large amount of refrigerant flows and the capacity as the air conditioning apparatus decreases. Therefore, a restrictor having a proper resistance of flow passage is employed.
In a refrigerating cycle in which the compressor operates with a constant speed of rotation, most of the refrigerating machine oil discharged from compressor 21 is separated by oil separator 29, and sucked into compressor 21 in the apparatus above. In the air conditioning apparatus mainly used now, an inverter compressor is used and the capacity control is achieved by controlling the operation frequency of the compressor. In this air conditioning apparatus, if the operation frequency of compressor 21 is low, the refrigerating machine oil discharged from compressor 21 and circulated in the refrigerating cycle is difficult to let return to the compressor 21 due to the low flow velocity of the refrigerant. Consequently, the amount of the refrigerating machine oil in compressor 21 could be insufficient, causing inadequate lubrication.